hi,
makes no difference whether the mod affects only singleplayer or also multiplayer. it would be quite easy to make such a mod but then you would not be able to join multiplayer servers anymore.
you'd have to make a backupcopy of the properties.u(thats the file you'd have to edit) and put it in when you want to play online.

I'm willing to do this sacrifice.
I'm only hoping that someone will make this mod.
Besides, I don't think I'm the only one that wants this kind of ammo mod.